1. Field of the Invention
The present invention relates to a powder of alloy particles that is particularly useful as a catalytic material and a method of manufacturing the powder. The powder of alloy particles according to the present invention is suitable for use as a catalytic material in electrode catalysts for fuel cells and catalysts for use in automotive emissions control, or as a biomolecule marker or drug delivery system (DDS) or other material used in medical treatment.
2. Background Art
Electrode catalysts for fuel cells and catalysts for use in automotive emissions control are actually provided in the form for practical use as catalytic equipment consisting of many different raw materials, but among the elements constituting this equipment, the properties of metal catalysts typified by Pt and Pd dominate the catalytic activity thereof. However, drops in this catalytic activity occasionally occur.
In electrode catalysts for fuel cells for example, electrical power is generated by the oxidation of hydrogen on the anode side and the reduction of oxygen on the cathode side, but since platinum is used as the electrode catalyst in most cases, these platinum catalysts suffer from poisoning due to strong CO adhesion. The main cause of this is that the hydrogen gas used on the anode side is often not expensive pure hydrogen gas, but rather hydrogen-enriched gas produced by reforming hydrocarbon fuels that may contain roughly 1% CO. Similar poisoning of platinum catalysts by CO adhesion also occurs in automotive emissions control catalysts, thus inducing a decrease in catalytic activity.
Studies of using platinum alloy catalysts as countermeasures against this problem of catalyst poisoning due to CO adhesion have been made. The use of alloys is intended to lower the CO oxidation potential. This is beneficial not only because the alloying is done with iron and nickel and other elements that are less expensive than the platinum-family metals but are also less susceptible to poisoning by CO adhesion, and also it is possible to use finer particles and improve the catalytic activity. Representative candidates include Pt—Fe-based and Pt—Ni-based nanoparticle powders.
However, the manufacture of such powders of fine alloy particles with greatly divergent standard electrode potentials is not necessarily easy. The standard electrode potentials of Pt and Pd are 1.50 V and 0.99 V, respectively, but the standard electrode potentials of Fe and Ni are −0.44 V and −0.25 V. If one tries to reduce metal ions of such greatly divergent standard electrode potentials, e.g. Pt ions and Fe ions, by the wet method using a reducing agent to precipitate an FePt alloy, the readily reduced Pt ions will be reduced first and as a result, either the Pt particles and Fe particles will precipitate separately or a core-shell structure in which the Fe precipitates around the Pt first precipitated may result, so it is difficult to produce alloy particles that are uniform on the atomic level.
Nanoparticles of FePt have recently attracted attention in the field of magnetic materials, and methods of producing them have been proposed in JP 3258295B (JP 2000-54012A; Patent Document 1) and in Science, Vol. 287, 17 Mar. 2000, pp. 1989-1992 (Non-Patent Document 1). These are methods of producing FePt alloy particles by performing the pyrolytic decomposition of iron pentacarbonyl simultaneously with the reduction of platinum(II) acetylacetonate by a polyhydric alcohol. The FePt particles obtained by these methods have a fcc (face-centered cubic) structure and a grain size of roughly, 2-5 nm, giving particles that are nearly spherical in shape.
Another method is that disclosed in the Japanese Journal of Applied Physics, Vol. 42, No. 4A, 1 Apr. 2003, pp. L350-352 (Non-Patent Document 2). Non-Patent Document 2 recites a method whereby tetraethylene glycol (TEG) is used as the polyol at the time of preparation of FePt nanoparticles by the polyol method, so when platinum and iron acetylacetonate are reduced at 300° C., FePt nanoparticles with the fct (face-centered tetragonal) structure are obtained as produced. The FePt particles are spherical and aggregated.